Chest X-Rays are among the most common films taken, being diagnostic of so many important problems.
Features that are typically examined on a chest X-ray
Every doctor will have a different approach to examining chest X-rays. A commonly used mnemonic for what to look for on a chest X-ray is: It May Prove Quite Right (but) Stop And Be Certain How Lungs Appear:
I = Identification (name, age, sex, indication for X-ray)
M = Markers (differentiate left from right - diagnose dextrocardia)
P = Position - the spinous process of T4 should be between the heads of the clavicle (if it isn't the body is rotated)
Q = Quality - is the film penetrated properly. In a properly penetrated film the vertebral interspaces should be visible behind the central (cardiac) shadow
R = Respiration - chest X-rays are typically done with full inspiration
S = Soft tissue - look for subcutaneous emphysema (suggestive of trauma), soft tissue swelling
A = Abdomen - look for free abdominal air (suggests penetrating trauma, peritonitis, or recent surgery)
B = Bone - look for fractures (these tend to be at the lateral aspects because of the mechanics - bending moment largest at lateral aspect)
C = Central shadow (cardiac silhouette) - greater than 50% of lateral distance in frontal view at the diaphragm suggests cardiac enlargement (usually secondary to heart failure) or a pericardial effusion). A widened mediastinum may suggest aortic dissection.
H = Hila (of the lungs) - can be affected in lung disease, malignant processes and infection (hilar lymphadenopathy).
L = Lungs - for consolidation, interstitial lung disease (reticular, nodular or reticulonodular), honeycombing, miliary pattern, granulomas, lung masses
The most common view is the PA (posterior-anterior) and is frequently done with a left lateral view (so one can identify the location of abnormalities in 3-D space). PA views are generally preferred to AP views (which are often done with mobile/portable X-ray equipment), but much less convenient in the ICU setting or when a patient cannot otherwise leave their bed. PA views are preferred because the central shadow is better defined, the magnification of the heart is reduced, and less of the lungs obscured by the heart/pericardial sac.
Decubitus - useful for differentiating pleural effusions from consolidation (e.g. pneumonia). In effusions, the fluid layers out (by comparison to an up-right view, when it often accumulates in the costophrenic angles).
Lordotic view - used to visualize the apex of the lung, to pick-up abnormalities such as a Pancoast tumour.
Expiratory view - helpful for the diagnosis of pneumothorax
A nodule is a discrete opacity in the lung which may be caused by:
Fluid in space between the lung and the chest wall is termed a pleural effusion. There needs to be at least 75ml of pleural fluid in order to blunt the costophrenic angle on the lateral chest X-ray, and 200ml on the posteroanterior chest X-ray. On a lateral decubitus, amounts as small as 5ml of fluid are possible. Pleural effusions typically have a meniscus visible on an erect chest X-ray, but loculated effusions (as occur with an empyema) may have a lenticular shape (the fluid making an obtuse angle with the chest wall).
Pleural thickening may cause blunting of the costophrenic angle, but is distinguished from pleural fluid by the fact that is occurs as a linear shadow ascending vertically and clinging to the ribs.
The differential for diffuse shadowing is very broad and can defeat even the most experienced radiologist. It is seldom possible to reach a diagnosis on the basis of the chest X-ray alone: high-resolution CT of the chest is usually required and sometimes a lung biopsy. The following features should be noted:
type of shadowing (lines, dots or rings)
reticular (crisscrossing lines)
nodular (lots of small dots)
rings or cysts
consolidation (diffuse opacity with air bronchograms)
It must be remembered that while the chest X-ray is a cheap and safe method of investigating diseases of the chest, there are a number of serious chest conditions that may be associated with a normal chest X-ray and other means of assessment may be necessary to make the diagnosis: